Land-cover change is driven by the natural climate change and human activities, in turn, the land-cover change also has impact on regional climate. In this paper, we produced a high resolution land cover datasets suited for model based on the high accuracy vegetation cover map, MODIS LAI datasets and the vegetation division map in China. In order to cognizing the impact of vegetation cover changes on regional temperature and precipitation over China, we conducted two sensitivity experiments numerical simulations (1999-2009) by using Regional Climate model RegCM4.5 coupled with the land surface model CLM4.5 under two different vegetation cover scenarios, which are driven by ERA-Interim reanalysis data produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). Meanwhile, we adopted two methods, surface heat balance analysis and atmospheric low circulation analysis to researching the mechanisms of air temperature and precipitation change and evaluating the pathways of vegetation cover changes, such as afforestation, affecting the regional air temperature and precipitation. The main conclusions of this study are summarized as follows:
(1) RegCM4-CLM is reliable for its ability to simulate the spatial distribution of air temperature and precipitation over China, and the simulated values are consistent with the observed values. Compared to the winter, the model performances on air temperature and precipitation are better in summer, and the accuracy of precipitation simulation is not as well as the temperature. Meanwhile, result shows an overall agreement between the observed and modelled inter-annual and annual changes in mean air temperature and precipitation over China. The tendency of simulated and observed temperature and precipitation changes are consistence in time scale, and the timing of simulated anomalies peaks are closely match the observed data. RegCM4.5 also is reliable for its ability to simulate the annual cycle of temperature and precipitation, and the inter-annual period can be well captured in the model.
(2) The impacts of vegetation cover changes on regional climate could be large in summer and small in winter and be different in different vegetation cover change types. An increase of the average 1999-2009 summer air temperature (warming range to above 0.5 ℃) and precipitation (wetting range to above 80 mm) were found in the south China where croplands was converted to forestland, which contrary to the conversion of grassland to forest land in northeast China, the average summer temperature were decreased(cooling range to reach 1 ℃). When the shrubland was replaced by grasslands (C3 Non Arctic Grass/Arctic Grass), although the impact of grassland on temperature are consistent in central China and the Tibetan Plateau, which brings about a decrease in summer temperature, but the C3 Arctic grass has a stronger effect on temperature than C3 Non Arctic grass with more temperature decreased (the minimum cooling value to reach -4.47 ℃) in the Tibetan Plateau. Different types of grassland also have different effects on precipitation. The increase of C3 Non Arctic grass resulted in an increase of the summer rainfall (the maximum wetting value to reach 105.17 mm) in central China, while the increase of C3 Arctic grass resulted in a decrease of the summer precipitation decrease in the Tibetan Plateau.
(3) The vegetation cover change can alter various physical characteristics of the land surface, including the land surface parameters such as roughness, albedo and leaf area index (LAI). All the changes in these parameters can influence the atmospheric low circulation and energy budget through the land-surface process, which has an effect on the air temperature and precipitation. In the South region of China for example, as a result of croplands being replaced by natural vegetation, summer and albedo decreased, roughness length increased, the sum of leaf area increased, and stomatal resistance increased. The summer albedo decreased which increased the solar radiation absorbed by the vegetation. The increased absorbed solar radiation was offset somewhat by increased longwave loss to the atmosphere so that net radiation (absorbed solar– net longwave) increased. This was balanced by an increase in vegetation sensible heat flux and a decrease in latent heat flux (canopy evapotranspiration). By analysis the energy budget in South region, it is necessary to reach a higher TX for make the short waves flux, the long waves, and the delivery of effective heat flux to a new balance. Moreover, there are significant changes in the near ground wind field in summer due to the vegetation cover change, and the changes of wind field are stronger in summer than in winter in terms of extent and scope. Changes in direction and speed of wind in typical vegetation cover change areas are significantly stronger than those in the surrounding area with unchanged vegetation. An anti-cyclonic circulation dominates in the Tibetan Plateau and central China, indicating that the summer monsoon intensity was generally weak, but it increased in the monsoon marginal regions such as southeastern China. Results indicate that near-ground circulation changes have a certain effect on regional precipitation. The research results can not only deepen the understanding of the relationship between vegetation and climate change, but also provide decision making for ecological environment construction, and for mitigating the environmental impact of regional climate change.